1. Field of the Invention
This invention relates to spread spectrum communication equipment and more particularly to a minimum shift keyed modulator.
2. Description of the Prior Art
Recent progress in coded, spread-spectrum communication systems has pointed up the need to avoid spillage of energy from one communication channel into the frequency band allocated for adjacent and nearby communication channels, such as tactical communication and navigation systems (TACAN) and other equipment. This is the principal reason for the increasing usage of minimum shift keyed (MSK) waveforms, also known as continuous phase shift modulated (CPSM) waveforms for direct sequence coding in spread-spectrum communication systems.
Continuous phase shift modulated spread-spectrum waveforms consists of a sequence of contiguous pulses, each of which is a short pulse at one of two different frequencies. They have the advantage of lower spectral side lobes, and thus lower cross-channel interference, than phase shift keyed (PSK) waveforms.
In a publication by W. R. Smith entitled "SAW Filters for CPSM Spread-Spectrum Communications", 1977 Ultrasonics Symposium Proceedings, IEEE Group on Sonics & Ultrasonics, Oct. 26-28, 1977 at Phoenix, Ariz., pp. 524-8, two methods are described for generating CPSM waveforms. The first method is by direct synthesis without surface acoustic wave (SAW) filters. In direct synthesis, the carrier portion of these waveforms is generated by an i.f. oscillator. A 90.degree. phase shifter provides the in-phase and quadrature components of the carrier frequency. A modulation frequency is obtained by narrow band filtering two of four phase outputs of a divide-by-four circuit which is driven by a square wave generator of period T. The output of each filter is coupled to respective mixer. One mixer has a second input coupled to the carrier frequency and the second mixer has a second input coupled to the quadrature phase of the carrier frequency. A spreading function or input code is stretched in time by a factor of two and split into two sequences, one consisting of the odd numbered chips and the other consisting of the even numbered chips. The odd and even numbered chips are coupled to the inputs of two additional mixers having the second input coupled to the output of in-phase and quadrature components of the modulated carrier. The output of the two additional mixers is summed to provide a single output. The message to be transmitted is usually superimposed by simple modulo-2 addition to the code at the input.
The second method for generating CPSM waveforms is by CPSM synthesis using a surface acoustic wave (SAW) filter. A spreading function or input coded pulse train is mixed with a carrier frequency in a mixer to provide a phase shift keyed signal. The phase shift keyed (PSK) signal has chips having duration T. The PSK waveform is introduced into a SAW filter, whose impulse response is given to a good approximation by H(T)= sin (2.pi.f.sub.2 t), for 0.ltoreq.t&lt;T and 0, otherwise.
The modulator with the SAW filter is obviously much simpler, smaller and less expensive than the direct synthesis circuit. Furthermore, the critical time alignment problems associated with the direct-synthesis circuit have virtually no counterpart in the SW filter approach.
It is therefore desirable to provide an MSK waveform by direct synthesis using digital circuitry.
It is further desirable to provide digital circuitry for generating MSK waveforms to avoid the use of SAW device which is subject to: temperature stability, the initial cost and frequency range.
It is further desirable to provide digital circuitry for the direct synthesis of MSK waveforms to avoid the limitations of frequency and phase stability associated with a local oscillator.